Abstract: Coke deposited within the gas passages of a dual tower type, fluidized bed apparatus for thermally cracking hydrocarbon oils is removed by combustion through contact with a stream of an oxygen-containing, high temperature combustion gas without a need to disjoint the apparatus. The apparatus is comprised of heating and cracking towers each adapted for containing a mass of fluidized solid particles continuously recirculating between the two towers. In one embodiment, all of the solid particles are discharged from the apparatus and a stream of the oxygen-containing combustion gas, produced in a combustion furnace connected to the heating tower, is allowed to pass through the gas passages.